Feedstocks containing higher molecular weight hydrocarbons are cracked by contacting the feedstocks under elevated temperatures with a cracking catalyst whereby light and middle distillates are produced. Typically, the yield ratio of light distillate (gasoline) to middle distillate (light catalytic gas oil) is dependent upon the conversion level, therefore to increase the make of middle distillate, a corresponding decrease in conversion must be experienced. Unfortunately, this decrease in conversion requires significant changes in operating conditions which can have a detrimental impact on gasoline octane quality, or a change in catalyst type which can be time consuming and costly. Furthermore, presently available techniques for lowering the conversion level in a cracking operation result in poor selectivity to the desired middle distillate product, and instead, lead to high yields of undesirable heavy, 650.degree. F.+ slurry oils. Therefore, with the current increase in demand for middle distillate fuels, it is desirable to have a modified cracking process available for quickly and reversibly changing from a maximum gasoline mode of operation to a maximum middle distillate mode of operation without lowering the octane number of the gasoline, to meet both seasonal and longer term fluctuations in the demand for distillate products.